MOLECULAR-BIOLOGY, GENETICS AND REGULATION OF NITRITE REDUCTION IN HIGHER-PLANTS

Nitrite reductase (ferredoxin:nitrite oxidoreductase, EC 1.6.6.1) carr ies out the six-electron reduction of nitrite to ammonium ions in the chloroplasts/plastids of higher plants. The complete or partial nucleo tide sequences of a number of nitrite reductase apoprotein genes or cD NAs have been determined. Deduced amino acid sequence comparisons have identified conserved regions, one of which probably is involved in bi nding the sirohaem/4Fe4S centre and another in binding the electron do nor, reduced ferredoxin. The nitrite reductase apoprotein is encoded b y the nuclear DNA and is synthesised as a precursor carrying an N-term inal extension, the transit peptide, which acts to target the protein to, and within, the chloroplast/plastid. In those plants examined the number of nitrite reductase apoprotein genes per haploid genome ranges from one (barley, spinach) to four (Nicotiana tabacum). Mutants defec tive in the nitrite reductase apoprotein gene have been isolated in ba rley. During plastidogenesis in etiolated plants, synthesis of nitrite reductase is regulated by nitrate, light (phytochrome), and an unchar acterised 'plastidic factor' produced by functional chloroplasts. In l eaves of green, white-light-grown plants up-regulation of nitrite redu ctase synthesis is achieved via nitrate and light and down-regulation by a nitrogenous end-product of nitrate assimilation, perhaps glutamin e. A role for phytochrome has not been demonstrated in green, light-gr own plants. Light regulation of nitrite reductase genes is related mor e closely to that of photosynthetic genes than to the nitrate reductas e gene. In roots of green, white-light-grown plants nitrate alone is a ble to bring about synthesis of nitrite reductase, suggesting that the root may possess a mechanism that compensates for the light requireme nt seen in the leaf.